It is common to carry out the patternlike control of an open-shed-double-lift dobby for controlling the heddle frames of a weaving machine during the shed standstill period. In the case of modern, high-speed weaving machines, however, the standstill times for this type of control are no longer sufficient, because weaving machines are already known which operate without shed standstill. In these weaving machines, the time of the shed standstill is no longer a weaving dependent function, but depends on the time needed for proper control of the dobby.
German Patent No. 1 106 704 discloses a dobby in which the center region of a differential lever can assume three operating positions. This so-called three-position dobby is typically used for weaving plush fabrics. The heddle frame must thereby cover, between two pick insertions or between two shed standstills, either the entire or half the distance from the lower to the upper shed position, or vice versa.
The classic three-position dobby is not suited for the desired goal of a dobby with an increased speed level.
Means and ways have already been developed for constructing the dobby, in connection with the weaving machine, so that a significant extension of the control time beyond the usual duration of the shed standstill is possible and can occur by means of a preselection system for the control. A different solution involving an extreme shift of the control into the times of the shed change is described in Swiss Patent No. 501 074 (corresponds to U.S. Pat. No. 3,703,195).
If, in such a dobby, the Hattersley System lifting units are replaced with the relatively new type of high-speed lifting units which have become known under the name rotation dobby, for example, those disclosed in Swiss Patent No. 372 004, Swiss Patent No. 473 253, Swiss Patent No. 517 192 (corresponds to U.S. Pat. No. 3,730,231) German Patent No. 957 648, and German OS Pat. No. 2 362 518, the disclosures of which are incorporated herein by reference, a very expensive and complicated mechanism is required, since each drive shaft of one of the four lifting units must carry out one full rotation per pick at a different time than that of the three other lifting units.
A goal of the invention is therefore a simple and economical arrangement in which the shed formation preferably occurs by means of rotation dobbies and in which a shed standstill is not needed for the control.